Indium antimonide (InSb) and indium arsenide (InAs) are widely recognized as being particularly useful as an active layer in magnetic field sensors such as magnetoresistors, Hall effect sensors (Hall sensors) and MAGFETS (magneto-transistors). Magnetic field sensors can be used in a variety of magnetic field sensing applications, including position and speed sensing, compass sensors, magnetic memory readout, and control of brushless motors.
Indium antimonide is particularly attractive for use as an active layer in magnetic field sensors, primarily because it has a high electron mobility. High electron mobility is important because it is primarily responsible for providing high magnetic sensitivity. On the other hand, indium antimonide has a low energy band gap, of about 0.18 eV at room temperature. This gives it an intrinsic carrier density that is large at room temperature. However, in addition, its intrinsic carrier density increases dramatically with increases in temperature. This tends to interfere with magnetic sensitivity.
Also, the conductivity increase with temperature is so significant that it is difficult to accommodate with the usual type of signal conditioning circuitry. Accordingly, it is typical to stabilize the carrier density in indium antimonide with respect to temperature variations by doping it n-type, typically within a factor of about 3 of 1.times.10.sup.17 cm.sup.-3. The n-type dopant is ordinarily tellurium, selenium, sulfur, silicon, tin or the like. This increases electron density to a higher level so that differences in intrinsic carrier density with temperature become insubstantial, or are more accommodateable with signal conditioning circuitry. On the other hand, the resulting higher conductivity material has relatively low electrical resistivity. This favors use of thin films of indium antimonide for magnetic field sensors.
For the case of indium arsenide, the temperature coefficient of resistance is lower than for indium antimonide because the energy gap is larger. An excess density of electrons, over the intrinsic density, can be obtained from a surface accumulation layer. Electrons in indium arsenide are known to accumulate near its surface where it contacts air. Alternatively, indium arsenide can also be doped n-type using silicon, germanium, tin, tellurium, selenium or sulfur. Whichever technique is used, it is preferable that the excess carrier density, when calculated on an areal basis, be on the order of 0.2 to 10.times.10.sup.12 cm.sup.-2.
In the past, thin slabs of indium antimonide were made by thinning down high quality bulk crystals. They were thinned down to slabs approximately 20 .mu.m in thickness. Thinner slabs were desired for reducing series resistance of sensors made from such slabs. However, thinner slabs were difficult to produce by thinning bulk crystals. This led to the more recent practice of growing thin epitaxial films of indium antimonide on insulating gallium arsenide (GaAs) substrates. Such epitaxial films are typically grown to a thickness of approximately 1 to 3 .mu.m, which increases sheet resistance (film resistivity divided by thickness) by an order of magnitude compared to thinned down bulk slabs.
It should be mentioned that a higher resistance device draws less current at a given voltage. It therefore dissipates less power for a given bulk voltage bias. Alternatively, it provides a larger output voltage for a given current bias. In any event, even films of only three micrometers in thickness still have too low a resistance for many applications.
In the case of magnetoresistors, a higher resistance is easily obtained by integrating many series connected rectangular magnetoresistor units into one sensor body. Typically, the rectangular magnetoresistor units are integrally formed in an elongated mesa or line of indium antimonide. In the mesa, they are integrally connected end to end, in series fashion. The mesa or line, i.e., the series connected magnetoresistor units, can be as long as needed to obtain the total resistance desired. The individual magnetoresistor rectangles are defined and connected in series by a plurality of spaced transverse "shorting bars". In such a construction, increased resistance in the resultant device is obtained by simply increasing the length of the indium antimonide mesa and the number of "shorting bars".
As indicated above, indium antimonide is of interest as a magnetic field sensor because it can be made with a high electron mobility. High electron mobility is obtained only if the indium antimonide is monocrystalline and indium antimonide crystal is of high quality. By that we mean that there are few crystal imperfections such as dislocations and the like.
High quality crystal structure in an epitaxial film is obtained if the substrate is a high quality crystal of the same type and approximate crystal size. In general, the better the match, the higher quality epitaxial layer can be grown. In addition, for highest magnetic sensitivity and lowest power consumption, the substrate should be electrically insulating. To satisfy these and still other requirements, prior magnetoresistor indium antimonide epitaxial fills have been typically grown on monocrystalline wafers of gallium arsenide (GaAs) or indium phosphide (InP). High quality epitaxial fills can be grown even though the gallium arsenide and indium phosphide lattice constants are smaller than that of indium antimonide. We know that others have studied films of indium antimonide grown on silicon. Silicon has an even smaller lattice constant than gallium arsenide or indium phosphide and has a significantly lower energy band gap. This might suggest inappropriateness for magnetic field sensors. However, the latter studies were for infrared sensing applications, where temperature is low and constant and where intrinsic substrate conduction at room temperature and high electron mobility is not of concern. Further, we are not aware that any practical infrared devices resulted from these studies.
As indicated above, magnetic field sensors should have a relatively stable and high electron mobility. Magnetic field sensors, particularly magnetoresistors, are subject to temperature cycling. Temperature cycling can be expected to degenerate a thin film in many properties, especially mobility. This is especially expected for temperature cycling of the extensive type experienced in automotive applications. Automotive sensor specifications, for example, require no significant change in sensor performance after extended temperature cycling from -40.degree. C. to +200.degree. C.
Magnetic field sensors have been made for such applications by growing indium antimonide fills on gallium arsenide or indium phosphide wafers. Substrate wafers of gallium arsenide or indium phosphide are attractive because of their relatively high energy band gap, which makes them insulating and of relatively high dielectric strength. On the other hand, gallium arsenide and indium phosphide wafers are quite costly, and availability is limited because of commercially limited applications.
We have surprisingly found that satisfactory magnetic field sensors can be made with indium antimonide films grown on wafers of elemental semiconductors such as silicon and germanium, especially silicon. We recognize that silicon has a greater lattice mismatch with indium antimonide than gallium arsenide or indium phosphide. We also recognize that silicon has a lower thermal coefficient of expansion than indium antimonide. We further recognize that silicon and germanium have a relatively low energy band gap. Nonetheless, durable films of indium antimonide have been made on silicon and germanium substrates that have relatively stable and high mobilities, even after automotive type thermal cycling.
Accordingly, despite factors leading one away from use of silicon and germanium as magnetoresistor substrates, we have found that they can be quite successfully used. Indium antimonide cannot only be grown on a silicon or germanium substrate but it can have fairly high mobility, even with the inherent lattice mismatch. The resultant mobility, though not the highest obtainable, is adequate for many magnetic field sensing applications contemplated in the automotive industry. On the other hand, to make a commercially successful magnetic field sensor, one must also overcome the basically objectionable higher conductivity and lower dielectric strength of the silicon or germanium substrate, as compared to gallium arsenide or indium phosphide.
We have found a variety of ways to overcome the fundamentally higher conductivity and lower dielectric strength of the silicon or germanium substrate. We provide many forms of electric field protection in the substrate. The electric field protection can be provided in a form that concurrently also reduces parasitic electrical conduction in the substrate, as will hereinafter be described in detail.
In addition, it should be mentioned that we have recognized that substrate wafers of silicon and germanium offer a significant cost advantage. Silicon and germanium wafers are less expensive and are available in larger sizes. The larger sizes provide a manufacturing cost advantage. In addition, silicon wafers are mechanically stronger than the gallium arsenide or indium phosphide wafers. This is attractive for commercial production operations, which increases yields of satisfactory wafers. Also, the stronger material permits use of thinner wafers, which is beneficial from a magnetic circuit point of view. In other words, thinner substrates can allow thinner "air gaps", or regions of the magnetic circuit in which the magnetic permeability is near unity.